The Ikaros gene family encodes DNA-binding proteins essential for lymphocyte differentiation and homeostasis. In mature T cells, the majority of Ikaros protein is associated with the DNA- dependent ATPase Mi-2beta in a nuclear complex possessing chromatin-remodeling activity. Ikaros is required for the relocalization of the Ikaros-Mi-2beta complex to replicating heterochromatin upon lymphocyte activation. Disruption of this process may account for the leukemic phenotypes manifested by Ikaros-deficient cells. Ikaros proteins contain sequence- specific DNA-binding activity which may regulate the appropriate targeting of this chromatin-remodeling complex. Chromatin remodeling has been recently linked to thymocyte lineage commitment and T helper phenotype determination. A more detailed understanding of the regulation of these Ikaros-Mi-2beta chromatin-remodeling complexes will address fundamental questions about the molecular controls of lymphocyte activation, differentiation, and proliferation. Several approaches will be used to characterize the function and regulation of Ikaros-Mi-2beta complexes during lymphocyte activation. First, Ikaros-Mi-2beta complexes from quiescent and activated T cells will be characterized to determine the relative distribution of Ikaros and their chromatin-remodeling and histone deacetylation activities. Second, mutations in domains critical for the assembly of Ikaros-Mi-2beta complexes will be identified and assessed for their effect on lymphocyte activation. Finally, the activity of Ikaros-Mi-2beta complexes in a chromatin-based in vitro transcription assay will be determined. This work will provide the basis to tie together recent advances in chromatin remodeling and lineage determination in the hemo-lymphopoietic system with the biochemistry of Ikaros-Mi-2beta complexes upon signaling through the TCR.